Power line communication, also known as Broadband over Power Lines (BPL), is communications using a power line such as electric wires for supplying commercial power as a transmission line and is generally performed as balanced transmission using parallel electric wires. In indoor power line communication, each communication apparatus is connected through a modem to an electric outlet to which two or three electric wires are connected. Both transmission and reception are executed between the communication apparatus connected to the electric outlet.
Since the indoor power line is not essentially laid as the transmission line, the power line is not suitable to communications in some cases. For example, if an electric apparatus is not connected to an electric outlet, the electric wires are in a state of a so-called open stub at the electric outlet and the communication quality is adversely affected. That is, because the electric outlet becomes an open end (high impedance), the input impedance characteristic of the power line to which a predetermined load is connected becomes unstable relative to the frequency. Consequently, reflection of a high frequency signal occurs, and thus a communication signal is degraded.
In a state in which a power plug of an electric apparatus is connected to an electric outlet, the input impedance of the electric apparatus is small in a high frequency and the electric wires are in a state of a so-called short stub state at the electric outlet. Consequently, reflection of a high frequency signal occurs in the portion of the electric outlet, and the communication quality deteriorates.
US2006/0214507 A1 describes terminating devices that are for being connected to an electrical outlet of an indoor power line and that have a plug for enabling an electric apparatus to be connected thereto. However, if an electric apparatus for power line communication (a modem, etc.) is connected to the terminating device, a communication signal of power line communication is absorbed in the terminating device and the communication efficiency is reduced. If a modem, etc., is connected to the electrical outlet adjoining (or close to) the electrical outlet to which the terminating device is connected, absorption of a communication signal also occurs, and a decrease in the communication efficiency cannot be avoided.
It is therefore an object of the invention to provide a terminating device for making it possible to conduct power line communication with suppression of the degradation of the communication quality without decreasing the communication efficiency.